The Strange Ingredients Found in Vaccines | BBC Future

“In 1925, Gaston Ramon embarked upon an experiment that even he described as… “interesting”.

A few years earlier, the French veterinarian had been trying out a new diphtheria vaccine on horses, when he made an accidental discovery: some animals reacted by developing nasty abscesses at the injection site, and these ones also tended to develop stronger immune responses. This got him thinking – what else could he add to the vaccine, to encourage this to happen?”